Cosmic ray and photon collisions

ive been researching cosmic rays, and came across this question that i can't quite work out. can anyone help?
what is a cosmic ray proton of Etot 10^9 Gev, collides head on with a low energy photon from remnant 3K balck body radiation in the universe.
if the photon has energy kT, can we work out the energy of the centre of mass for the interaction? if we can, could we then calculate the energy of the photon in the reference frame where the proton is at rest and compare these?!?!?
any idea's?